A line interface circuit couples a telephone line to a telephone switching system. Among other things, it serves to provide both AC and DC power to the telephone line to operate the communication equipment (e.g., telephone) that is connected to the line. This function of supplying the DC power is performed by the battery feed circuit of the line interface circuit.
A variety of battery feed circuits have been used over time. The most common design, shown in FIG. 1, employs a large primary inductor 100 coupled to a source of AC signals and with two secondary windings 101, 102 (e.g., a transformer), one connected from the tip lead T of the telephone line to ground and the other connected from the ring lead R through a current limiter 110 to a power supply V1 (e.g., a battery). The two windings are closely coupled through a capacitor 111, whereby a high impedance is presented to differential signals on the tip and ring leads of the telephone line and a low impedance is presented to common-mode (longitudinal) signals. The load (e.g., a telephone terminal and its DC-to-DC converter circuit) seen by the battery feed circuit across tip and ring leads T and R is largely a capacitive load. Capacitive loads create large start-up currents when power is initially applied to them. The function of current limiter 110 is to limit start-up (in-rush) current surges.
In order for current limiter 110 to not interfere with the proper operation of the load, the current limit must be at least as large as the maximum operating load current. But while the voltage drop across the current limiter is relatively small during normal operation, almost the entire voltage produced by source V1 is seen as a voltage drop across the current limiter during start-up and during short-circuit faults on the telephone line. Yet at the same time, the impedance of the current limiter must be kept small in order for source V1 to supply to tip and ring leads T and R the constant-voltage feed required by digital telephone lines. Consequently, the current limiter dissipates relatively a lot of power during start up, and therefore must be robust and bulky to handle that power dissipation. This results in the current limiter being rather expensive.
To avoid the large power dissipation at start-up, some current-limiter designs employ power sequencing. Power sequencing is involves the use of a series of current-source stages of decreasing impedance which are switched on in sequence as the voltage drop across the current limiter decreases. Thus, when the voltage drop across the current limiter is large, the current is supplied through the high-impedance stage, whereby the current is relatively small and therefore the power dissipation is also relatively small. As the voltage drop decreases, lower-impedance stages are switched on, whereby the current flow is increased, but because the voltage decreases at the same time as the current increases, the power dissipation stays relatively small. While effective in limiting power dissipation, such power-sequences are rather complex and expensive.